Q C D Lance Dixon 40 th SLAC Summer Institute July 27 2012 Caveat Emptor Im not a historian Im a theorist I dont promise to be fair and balanced I was only there for the last 20 years ID: 178146
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Slide1
40 Years of
QCD
Lance Dixon 40th SLAC Summer InstituteJuly 27, 2012Slide2
Caveat Emptor
I’m not a historianI’m a theoristI don’t promise to be “fair and balanced”I was only “there” for the last 20 years
L. Dixon 40 years of QCDSSI40 July 27, 20122Slide3
L. Dixon 40 years of QCD
SSI40 July 27, 20123
The Times They Were A’Changing
In the 1960s there was no QCD, no
Lagrangian
or Feynman rules for the strong interactions.
Instead there was a baffling array of
“elementary” hadrons:
p, n,
L, S, X, p,
K,
h, r, w,
...
There were symmetries to group them:
-
isospin
– SU(2)
- “the eightfold way” – SU(3) (approximate) PCAC – spontaneously broken axial SU(2) or SU(3) leading to light Nambu-Goldstone bosons: p, KSlide4
Symmetries could be accounted for by having an SU(3) triplet representation of
u,d,s quarks (aces), with baryons ~ mesons ~ L. Dixon 40 years of QCD
SSI40 July 27, 20124
“Three quarks for Muster Mark”
- J. JoyceSlide5
Quark model predictions confirmed
L. Dixon 40 years of QCDSSI40 July 27, 2012
5
BNL
1964
uuu
ddd
sss
uds
But what about Fermi statistics?Slide6
Where were the quarks?
Why did only certain SU(3) representations appear? (“triality zero”)Where was the triplet itself?L. Dixon 40 years of QCD
SSI40 July 27, 20126Slide7
Were quarks even real?
“We construct a mathematical theory of the strongly interacting particles, which may or may not have anything to do with reality, find suitable algebraic relations that hold in the model, postulate their validity, and then throw away the model. We may compare this process to a method sometimes used in French cuisine: a piece of pheasant meat is cooked between two pieces of veal, which are then discarded.” - M. Gell-Mann
L. Dixon 40 years of QCDSSI40 July 27, 20127Slide8
Quarks were real!
L. Dixon 40 years of QCD
SSI40 July 27, 20128
SLAC MIT 1969Slide9
Fast forward 30 years
L. Dixon 40 years of QCD
SSI40 July 27, 20129
HERA
ZEUS
H1
HERA
SLAC
SLAC
MIT
Now one can easily “see” the struck quarkSlide10
Precision PDFs
From HERA and other data, we now know where the quarks “are” (in x) to a few percent
Essential input for all LHC predictionsL. Dixon 40 years of QCDSSI40 July 27, 201210Slide11
DIS @ SSI19, 1991
L. Dixon 40 years of QCDSSI40 July 27, 201211Slide12
Quarks were almost free
L. Dixon 40 years of QCDSSI40 July 27, 2012
12
At large
Q2, slow evolution with
Q
2
=
Bjorken
scaling
Justified by current algebra in infinite
momentum frame for proton (1969)
“A more physical approach into what is
going on is, without question, needed.”
-J.
BjorkenSlide13
Short-distance cross section
predictable using
perturbative QCD
L. Dixon 40 years of QCD
SSI40 July 27, 2012
13
Enter the
Parton
Model
At short distances,
quarks
and
gluons
(
partons
)
in
proton
are almost free. Sampled “one at a time
”
The “
femto
-universe”
s
ize
-1
= factorization scale
m
F
Parton
distribution
functions
(from experiment)
r
enorm
. scale
g
*
Drell - Yan 1970Slide14
First quantitative predictions
for hard pp collisionsL. Dixon 40 years of QCDSSI40 July 27, 2012
14
Drell-Yan 1970
“We will not speculate here on the presence of such a bump.”
Data from Christenson, Hicks, Lederman et al.
“The cross section varies smoothly … and
exhibits no resonant structure.”
Later: “Any apparatus that can
convert [a] towering peak into
this mound of rubble should be
proscribed by SALT talks.”
- L.
LedermanSlide15
Fast forward 40 years
L. Dixon 40 years of QCD
SSI40 July 27, 201215
Drell-Yan process still used to look for new particles at
hadron colliders. Standards for theorists speculating about bumps have changed.Slide16
L. Dixon 40 years of QCD
SSI40 July 27, 201216
But why were the quarks almost free?Asymptotic FreedomGluon self-interactions make quarks almost free, make
QCD
calculable at short distances:
g
s
2
/(4
p
) =
a
s
(
m
)
0
asymptotically as
m
∞
Quantum fluctuations of
massless
virtual particles polarize vacuum
Gross,
Wilczek
,
Politzer
(1973)
QED: electrons screen charge (
e
larger at short
distances, large
m
)
Non-
Abelian
gauge theory (
Yang, Mills (1954)):
gluons
anti-
screen charge (
g
s
smaller at short distances)
e
> 0
g
Slide17
L. Dixon 40 years of QCD
SSI40 July 27, 2012
17Fast forward 40 years
as
, and its running with Q
, now
known
precisely
from many experiments (and
high-order theory
)
confining
calculable
Bethke
van
Ritbergen
,
Vermaseren,
Larin
(1997)Slide18
The
LagrangianL. Dixon 40 years of QCDSSI40 July 27, 2012
18
n
f
spin ½ matter (quarks) in 3 of SU(3) color
coupled to spin 1 vector fields (gluons) in 8 (
adjoint
)
neglecting quark masses, only one
dimensionless
parameter at classical
level
Gauge theories renormalized by ‘t
Hooft
and
Veltman
(1972)
Fritzsch
, Gell-Mann,Leutwyler (1973)Slide19
QCD action
soon defined nonperturbatively on the latticeL. Dixon 40 years of QCD
SSI40 July 27, 201219
Wilson, 1974
Quarks shown to be confined
– in the strong-coupling
approximation
Zanotti
,
ICHEP2012Slide20
L. Dixon 40 years of QCD
SSI40 July 27, 2012
20Fast forward 35 years (or 1 petaflop-year)
Zanotti
,
ICHEP2012
Strong-coupling
QCD understood
quantitatively (for
static quantities)Slide21
The November Revolution
L. Dixon 40 years of QCDSSI40 July 27, 2012
21
Heavy quarks beyond any doubt
(well at least no doubt by 1976)
Charm: a weak
iso
-partner for the strange quark, needed for:
1) GIM mechanism to suppress
flavor-changing neutral currents
2) so Shelly Glashow did not have to eat his hat
Coronation of the Standard Model
(over the next few years)Slide22
The New Spectroscopy*
L. Dixon 40 years of QCDSSI40 July 27, 2012
22 The hydrogen atom of the strong interactions Theorists could imagine using old-fashionednonrelativistic potential techniques
E. Bloom, SSI 1981
Crystal Ball
F. Porter, SSI 1981
V(
r
)
r
*Title of H.
Harari
and G. Trilling SSI 1975 lectures
Dine,
Sapirstein
in M.
Chanowitz
lectures at
SSI 1981Slide23
Fast Forward 30 years
The New New Spectroscopy
L. Dixon 40 years of QCDSSI40 July 27, 201223
Mizuk
, ICHEP2012
?
?
?
?
d
?
X(3872) also
CDF, D0,
LHCb
, CMS
not all states confirmed
not from cc potential models!
_Slide24
Jets in the early days
L. Dixon 40 years of QCDSSI40 July 27, 2012
24G. Hanson et al. (1975)
MARK I @ SPEAR
[of
y
,
y
’
,
t
,
D
fame]
due to transverse
beam polarization
6.2
GeV
7.4
GeV
6.2
GeV
7.4
GeV
3.0
GeV
l
i
=
eigenvalues
of EM tensor
You would never
recognize a jet in the event display!Slide25
Fast forward 35 years
Jets very visible everywhere at hadron colliders
L. Dixon 40 years of QCDSSI40 July 27, 201225
8 jets …
2 jets
CMS
V. SharmaSlide26
Jets span a massive dynamic range
L. Dixon 40 years of QCDSSI40 July 27, 2012
26
Excellent agreement with NLO QCD predictions (Ellis,
Soper 1990) But NNLO would be even better (anticipated breathlessly)
pp
jet +
X LHC
pp
jet +
X
Tevatron
_Slide27
L. Dixon 40 years of QCD
SSI40 July 27, 2012
27
LHC Data Dominated by Jets
new physics
Every process shown also with
one more jet
at
~ 1/5 the rate
Need
accurate production rates
for
X
+
1,2,3,…
jets
in Standard Model
Jets
from quarks and gluons.
q,g
from decay of
new particles
?
Or from
old QCD
?Slide28
Where was
the gluon?First sightings:e
+e-
(9.46
GeV
)
ggg
PLUTO at DORIS/DESY (1979)
e
+
e
-
(
20-30
GeV
)
qqg
3 jets
TASSO, PLUTO, MARK J, JADE
at PETRA/DESY (1979)
L. Dixon 40 years of QCD
SSI40 July 27, 2012
28
_Slide29
Fast forward 25 years
L. Dixon 40 years of QCDSSI40 July 27, 201229
Still hard to tell gluons from quarks Do it statistically
using width of jets,
“jet shape” – fraction of energyin smaller cone
with
Kinematics (p
T
) selects
gluon-rich
or gluon-depleted
samples
Works in pp using
p
T
jet
as tag
CDF (2005)
And in
ep
at fixed
p
T
jet
using p
T
order
of jet as tag
(2007)
k
T
algorithm
g*Slide30
Yet heavier quarks:
b, tL. Dixon 40 years of QCD
SSI40 July 27, 201230Lederman et al. (Fermilab
, 1977)
(9.46
GeV
)
m
+
m
-
D0
CDF
Fermilab
(1994)Slide31
Fast forward 18 years
Copious top (and bottom) samples at LHCTheoretical challenge: Describe top quark production cross section at hadron colliders at NNLO in QCD.Recently achieved for Tevatron
(qq initial state easier) cuts theor. uncert. in halfL. Dixon 40 years of QCD
SSI40 July 27, 2012
31
_
Bärnreuther
,
Czakon
,
Mitov
(2012)Slide32
QCD and Higgs
Dominant Higgs production cross section is a QCD loop effect:
Georgi, Glashow, Machacek, Nanopoulos (1978)L. Dixon 40 years of QCDSSI40 July 27, 2012
32
Fast forward 34 years:
Lectures by F. Petriello, V. Sharma, M. Peskin, …Slide33
L. Dixon 40 years of QCD
SSI40 July 27, 2012
33F. Krauss
~2009
QCD Monte Carlos
Have also come a very long way since ~ 1977
Indispensable tool for experimental analysis
Now regularly incorporate
LO and even
NLO
QCD
corrections for many processes
G. Fox,
SSI 1981
Gehrmann,
Höche
,
Krauss,
Schönherr
,Siegert, 1207.5031
e
+
e
-
n
partons
merged NLO
sampleSlide34
Computational
perturbative QCD beginsL. Dixon 40 years of QCDSSI40 July 27, 2012
34
gg
ggg
at tree level (LO)
Squared-amplitude technique & Feynman diagrams Gottschalk,
Sivers
(1979)Slide35
Remarkably simple QCD tree amplitudes found in 1980s
… simplicity was secretly due to
N=4 SYMNow recycle this simplicity at loop level in QCD
Parke-Taylor formula (1986)
Now compute
helicity
amplitudes directly
L. Dixon 40 years of QCD
SSI40 July 27, 2012
35Slide36
L. Dixon 40 years of QCD
SSI40 July 27, 2012
36Back to the1960’s:Revenge of the Analytic S-Matrix
Bootstrap program for strong
interactions: Reconstruct scattering amplitudes directly
from
analytic
properties: “on-shell” information
Landau;
Cutkosky
;
Chew
, Mandelstam;
Eden,
Landshoff
,
Olive,
Polkinghorne
;Veneziano; Virasoro, Shapiro;
… (1960s)
Analyticity
fell out of favor in 1970s with the rise of
Q
C
D
& Feynman
rules
Now
resurrected
in
on-shell methods
for
computing
amplitudes in
perturbative
Q
C
D
–
as
alternative
to Feynman diagrams
!
Perturbative
information now assists analyticity.
Poles
Branch cutsSlide37
On-shell methods
many more processes @ NLOL. Dixon New Directions in Scattering TheoryICHEP Melbourne 2012 July 11
37Slide38
Conclusions
QCD is a remarkable theory:It is ultraviolet complete, with a nonperturbative
definition and no parameters in need of fine tuning (save q)It gives us a new way of thinking about the structure of matter: constituents that can never be isolatedMany of its principles have been copied in theories of physics beyond the Standard ModelThe boundaries of QCD in kinematics and precision are
continually being pushed, experimentally and theoretically
Our improved understanding of QCD
has
been,
and will
remain,
essential
to Higgs studies and in the search for new physics at the LHC and beyond
L. Dixon 40 years of QCD
SSI40 July 27, 2012
38Slide39
Happy 40
th Birthday QCD and S
SI!L. Dixon 40 years of QCD
SSI40 July 27, 2012
39